Aerospace vehicles that traverse, exit, and enter the atmosphere of the Earth travel at high velocities, and as a result, their exterior aerosurfaces, and to some degree their substructure, experience extreme thermal conditions. Heatshield materials are materials having high thermal conductivity to ensure that heat is quickly conducted away from potential hot spots on the heatshield. Heatshield materials also have high specific heat capacity so that the temperature increase of the material after absorbing heat is lower than that of many other materials. When applied to surfaces of an aerospace vehicle, heatshield materials form a heatshield that protects and insulates the structure subjected to thermal stress by the extreme thermal conditions.
The thermal stress management technique using heatshield materials that ablate under high temperatures has been used for a variety of applications since the early 1930s. Heatshield materials were used in early rocket systems for nose cap protection and have also been used as re-entry heatshields on the Gemini and Apollo space vehicles, and further on many modern rocket nozzles.
Common cork based heatshield materials include cork epoxy, cork phenolic and cork silicone. The combustion of cork and phenolic resin to form weakened char is an important failure mode of cork phenolic heatshield materials. When the material is exposed to high heat flux and oxygen from ambient atmosphere, the cork-based heatshields quickly char and begin burning. Once ignited, the heatshield materials will continue to burn even after the external heat source is turned off. As the cork phenolic heatshield ablates, the surface of the heatshield will form char with cracks, the size of which increases with time. Eventually the remaining material will break and erode away due to the mechanical load or aerodynamic shear.
Many of these materials, although suitable for use in the aforementioned applications, have handling and longevity (shelf life) issues that preclude application on a system that is subjected to frequent handling and that may be stored for extended periods of time prior to use. A typical launch vehicle may sit on the launch pad for days prior to flight (or stored somewhere for years), and often the heatshield materials can absorb a significant amount of moisture if left unprotected. Mildew and fungus (mold) can accumulate in and on the cork material of the heatshield, which may affect mechanical and thermal properties of the heatshield. Attempts have been made to coat the cork-based resin with an antibiotic such as para-nitrophenol (NO2-Ph-OH). However, this compound is highly water soluble (16 g/L) which correlates with its high toxicity, and is not permitted for use in aircraft in both the United States and the European Union.
Aerospace vehicles may also be protected from moisture (and mold) by coating heatshield surfaces with one or more sealant layers. These additional layers increase the weight and cost of the vehicle components and aerospace vehicles as a whole. These additional layers also increase the amount of time to manufacture such components and vehicles as a whole.
Therefore, there is a need in the art for new and improved heatshields and heatshield materials.